Light reflecting from an object's surface at an angle of reflection to the surface equal to the angle of incidence to the surface is known as the specular angle of reflection, or simply specular reflection or the specular direction. Gloss is associated with the capacity of a surface to reflect more light in the specular direction as compared to other directions. Material surfaces that are not perfectly smooth are assumed to have a microscopic level of detail which consists of a statistically large distribution of microfacets.
Most light reflected from a material surface arises from the following four phenomena:
(a) Light waves which specularly reflect off a planar surface significantly larger than the wavelength; PA1 (b) Light waves which go through at least two multiple specular reflections amongst multiple microfacets; PA1 (c) Light waves which penetrate into the top layer of the material surface, multiply refract, then are reflected back out; and PA1 (d) Light waves which diffract from interfaces with surface detail about the same size or smaller than the wavelength of the incident light. PA1 1. The use of polarizing filters to generate two images which are subtracted on an absolute pixel by pixel basis and corrected for falloff. The resulting difference image is considered to be a gloss image; PA1 2. A CCD camera to measure gloss with multi-shutter speed capability in order to gain dynamic range; PA1 3. A sample table with height adjustment to maintain measuring geometry regardless of sample height; PA1 4. A monitored light source that senses changes in light level relative to the level measured during calibration so as to correct absolute gloss calibration; PA1 5. The derivation of a gloss image to be used in conjunction with gloss measurements; PA1 6. A pure difference image used to correct for image falloff characteristics; PA1 7. Differentiating between shadowed areas from those areas containing specular reflections by using a threshold limitation; PA1 8. Grouping gloss pixels by amplitude in order to generate a Gloss Profile Summary; PA1 9. Correcting for errors during operation of the gloss measurement by utilizing calibration standards and calibration routines, including corrections for black level offsets, gain factors and geometry changes; PA1 10. Utilizing a light source and camera pattern correction algorithm to provide illumination/sensing field flattening, thus improving system accuracy; and PA1 11. Method and apparatus permitting measuring an object's gloss regardless of shape, texture and color.
The component of reflection which arises from phenomenon (a) is called the specular component of reflection. The angle that the incident light wave intercepts the test surface normal is called the specular angle of incidence. The component of reflection arising from phenomena (b) through (d) is called the diffuse component of reflection. If a beam of unpolarized light strikes a non-metallic surface at a specific angle, the reflected beam will be partially or completely linearly polarized. The degree of polarization depends upon the incident angle and the refraction index of the reflecting surface. The angle of which the degree of polarization is 100% is defined as the maximum polarization or Brewster angle.
Unpolarized light has equal magnitude polarization components in all directions. However, unpolarized light becomes partially polarized upon specular reflection. It is known that the polarization component perpendicular to the specular plane is larger than the magnitude of the polarization component parallel to the specular plane. It also is known that for a single specular reflection at an oblique angle, the magnitude of the perpendicular polarization component is not as attenuated as the magnitude of the parallel polarization component.
Over many years various equipments and/or methodologies have been devised or proposed to measure the visual gloss of various items. Reference may be made, for instance, to the following U.S. Pat. Nos. of interest: 3,999,864; 4,613,235; 4,746,805; 4,750,140; 4,886,355; 4,945,253.
Mutter U.S. Pat. No. 3,999,864 shows a gloss measuring instrument which transmits multiple beams of light to a fiber optic bundle to direct the light towards a sample at various angles of incidence. Light reflected from the sample is provided to a photodetector at different wavelengths that are a function of the angle of incidence of the light onto the sample. The photodetector develops signals indicative of the intensities of the wavelengths of light reflected from the surface sample which, along with a gloss standard, are used to compute a gloss measurement of the sample. Suga U.S. Pat. No. 4,613,235 discloses a gloss measuring system directing parallel light rays against a surface to measure gloss at a particular angle of incidence and corresponding angle of reflection. The central portion of the reflected light is blocked so that only the light diffusedly reflected from the surface at the angle of reflection is used to determine the gloss of the surface. Stapleton U.S. Pat. No. 4,746,805 discloses a gloss meter which includes a light source illuminating a sample surface and a motor driven chopper blade for interrupting the illumination and causing a reflection of the edge of the blade in the finish of the surface. The detector output is analyzed to determine the rate of change of the detector signal which is integrated to determine the gloss of the surface. Asano, et al. U.S. Pat. No. 4,750,140 shows a method and apparatus for determining glossiness of a surface based on the visible feeling by the light reflection intensity distribution of a surface. The gloss of the surface is determined by measuring the ratio of the specular reflection intensity to the total reflection intensity of the surface, plotting the measured ratio in relation to a predetermined curve, and determining which of the predefined curve sections the measured ratio corresponds to. Keane U.S. Pat. No. 4,886,355 shows an instrument for measuring gloss by detecting light specularly reflected from an illuminated sample surface. The detected intensity of the specular reflection provides an indication of the gloss of the sample surface. Frohardt U.S. Pat. No. 4,945,253 shows a device which optically measures the gloss of a surface using diffused light reflected from the surface.
None of the prior techniques has proven successful when measuring the gloss of an irregularly shaped substance of varying colors. In fact, manufacturers of present day gloss measuring equipments do acknowledge this technology limitation and continue to seek a solution to this longstanding problem.